As demand for consumer electronic products such as smart phones grows vigorously, electronic compasses utilizing magnetic field sensing technology have eagerly been required. Hall sensors and ARM (anisotropic magneto-resistor) sensors may be the most popular magnetic field sensors, but both suffer from low sensitivity to magnetic fields and consume a large device area. Using a tunneling magneto-resistor (TMR) as magnetic field sensor has the benefits of higher sensitivity and lower device area than the Hall sensor and ARM sensor. However, the TMR merely performs single-axis sensing for magnetic fields and lacks linearity to respond to magnetic fields. Thus, it is not easily applied to electronic compasses.
A typical TMR for a magnetic field sensor 100 is shown in FIGS. 1A to 1B. The TMR 100 includes a bottom plate of conducting metal, servings as a bottom electrode 102 formed on a substrate 90, an MTJ (Magnetic Tunneling Junction) device 110 formed on the bottom electrode 102 and a top plate of conducting material, serving as a top electrode 106 formed on the MTJ device 110. From the structure pattern of the MTJ device, one can define a cross having an intersection at a center, wherein the longer length is called a major axis 101 and the shorter length is called a minor axis 103. A line called an easy-axis 180 is collinear with a major axis 101. The MTJ device 110 includes a pinned layer 112, a tunneling layer 115 and a free layer 116, in which the MTJ device 110 is sandwiched between the bottom electrode 102 and the top electrode 106, for example. The pinned layer 112 is made of magnetic material formed on the bottom electrode 102 and has a first pinned magnetization 114, parallel to a pinned direction. The tunneling layer 115 of non-magnetic material is formed on the pinned layer 112. The free layer 116 of magnetic material is formed on the tunneling layer 115 and has a first free magnetization 118, initially parallel to the easy axis 180.
Before or after the MTJ device is formed (i.e. magnetic thin film stacking and pattern etching), the pinned direction is set by applying a field thereto during an anneal process. After the anneal process, the pinned direction will be parallel to the direction of the applied field, and the free magnetization tends to be parallel to the easy-axis due to the shape anisotropy. Therefore, the magnetic field sensing direction of the TMR is perpendicularly to the easy-axis 180. Additionally, the magnetic film typically is material of horizontal polarization and suffers from a very strong demagnetization field, which confines the activities of magnetizations of free and pinned layers, all of which are in-plane of the magnetic film. Namely, it is easy to rotate the free magnetization on the horizontal plane but the free magnetization hardly stands perpendicular to the plane of the magnetic film. Consequently, the typical structure of the TMR is only available for a single axis magnetic field sensor.
FIGS. 2A˜2B are drawings, schematically illustrating the cross-sectional view and the top view along the line at the easy-axis of a mutual supplement tunneling magneto-resistor, called a MS-TMR, having good linearity to respond to magnetic fields. In FIGS. 2A˜2B, the MS-TMR 150 includes a bottom electrode 102 of conducting material on a substrate 90 and a top electrode 106 of conducting material and first and second MTJ (Magnetic Tunneling Junction) devices 110a, 110b, disposed between the bottom electrode 102 and the top electrode 106. The first and second MTJ devices 110a, 110b have a collinear easy-axis 180. The first MTJ device 110a includes a pinned layer 112a of magnetic material formed on the bottom electrode 102 and has a first pinned magnetization 114a parallel to a pinned direction 140 which has an angle of 45 degrees to the easy-axis 180. A first tunneling layer 115a of non-magnetic material is formed on the first pinned layer 112a. A first free layer 116a of magnetic material is formed on the first tunneling layer 115a and has a first free magnetization 118a initially parallel to the easy-axis 180. The top electrode 106 connects to the first free layer 116a. 
The second MTJ device 110b has the same structure pattern and film stack as the first MTJ devices 110a. The second MTJ device 110b includes a second pinned layer 112b of magnetic material formed on the bottom electrode 102 and has a second pinned magnetization 114b also parallel to the same pinned direction 140. A second tunneling layer 115b of non-magnetic material is formed on the second pinned layer 112b. A second free layer 116b of magnetic material is formed on the second tunneling layer 115b and has a second free magnetization 118b initially parallel to the easy-axis 180 but anti-parallel to the first free magnetization 118a. The top electrode 106 connects to the second free layer 116b. 
A metal line 108 passes across the first and the second MTJ devices 110a, 110b and a set current ISET can be applied to generate a first ampere field around the metal line. Here, the metal line 108 passes over the first and the second MTJ devices, for example; but it also can pass below the first and the second MTJ devices. The first ampere field applied on the first and second MTJ devices 110a and 110b respectively, are parallel to the easy-axis 180 but directionally opposite, thus the first and second free magnetizations 118a, 118b are set to be anti-parallel.
A magnetic field sensing circuit may comprise a TMR sensing unit and a TMR reference unit. The TMR sensing unit and TMR reference unit are of the same structure as depicted in FIGS. 2A and 2B. When the magnetic field sensing circuit activates to sense an external magnetic field, another ampere field is first applied to the TMR reference unit by applying a tuning current in the metal line above or below the first and second MTJ device of the TMR reference unit. The ampere field makes the free magnetizations of the MTJ devices in the TMR reference unit fixed to the easy-axis, and equivalently the TMR reference unit is not influenced by the external magnetic field to be sensed, thereby fixing the conductance (or resistance) of the TMR reference unit. Large tuning current is required to generate the magnetic field for operation of the TMR reference unit, and therefore additional power consumption is inevitable. A novel TMR reference unit operating without additional power consumption is desired.